Quantitative investigation was performed to understand effects of length scales (film thickness) on grain growth and corresponding fatigue damage behaviors in the nanocrystalline Au thin films on flexible substrates with a film thickness ranging from 930 nm to 20 nm. In thicker films (h >= nm), abnormal grain growth happened due to the local high stress, while in thinner films uniform grain growth occurred. Such length dependent grain growth mechanisms was found to be associated with the film strength and the film thickness. Consequently, the main damages in thicker films exhibited the applied load range-related competition between the intergranular cracks and the intragranular cracks along the localized slip. In the thinner film, fatigue damage is associated with GB-related behaviors, such as intergranular cracking and deformation twinning.